Power supply units convert the AC power from the building into the correct voltage DC and deliver power to the components of a PC.
As time progresses, computer parts are becoming more energy efficient, thus the need for a high wattage PSU is slowly decreasing.
Power supplies are rated by numbers and colour using the voluntary “80 Plus” certification. This is a global standard that has been adopted by most manufacturers. Put simply, the higher the rating, the more energy efficient the power supply is, meaning money saved on power bills.
The order of preference is 80 Plus Platinum > 80 Plus Gold > 80 Plus Silver > 80 Plus Bronze > 80 Plus.
For high end systems, and for computers that are on 24/7, aim for a gold rated PSU.
For the everyday office worker or casual gamer doing a few hours a day, there’s nothing wrong with an 80 Plus or bronze/silver-rated PSU.
In terms of wattage, the majority of office PCs and gamers should use a power supply that is under 600W. It’s much better to invest in a higher rated PSU with lower wattage than the other way around. Only people running high-specced gaming systems with two graphics cards or enterprise parts would need to venture 750W or above.
Fully modular power supplies are excellent for those who want to perform neat cabling in their PC. This allows you to remove all unused cables. Non-modular or semi-modular PSUs will not allow you to remove some cables.
While power supplies are a boring component of a PC, it's one you want to get right (for reliability and longevity) as they can last a long time and possibly be re-used in future systems.